(1) Field of the Invention
This invention relates to a process for preparing a membrane for use as a battery separator.
(2) Description of the Prior Art
A graft film prepared by graft polymerizing polyethylene film with acrylic acid has been conventionally used for a separator membrane of battery. U.S. Pat. No. 3,427,206 discloses a good electrically-conductive film available for a separator membrane of a battery prepared by immersing a polyethylene film in a solution comprising aromatic hydrocarbon solvents such as benzene and acrylic acid and, optionally, a small quantity of carbon tetrachloride and irradiating with .gamma.-rays. However, the method of U.S. Pat. No. 3,427,206 suffers from the defects that a homopolymer of acrylic acid is apt to be formed and it is difficult to obtain a polyethylene film with acrylic acid grafted uniformly thereon. That is to say, acrylic acid is easily polymerized by irradiation with ionizing radiation, and therefore, when the acrylic acid is irradiated with ionizing radiation in the presence of polyethylene film, the acrylic acid is polymerized in the solution phase and is spent uselessly before graft reaction with the polyethylene film occurs. The homopolymer of acrylic acid formed in the invention disclosed in U.S. Pat. No. 3,427,206 brings about not only the loss of acrylic acid, but also, because the homopolymer adheres relatively strongly, it is very troublesome to remove from the film finally obtained. Furthermore, the polymerization of acrylic acid proceeds at an increasing rate; when the graft reaction occurs at certain points of the polyethylene, the acrylic acid easily diffuses into these parts to cause acceleration of the graft reaction and consequently it is very difficult to prepare a homogeneous graft film.
In order to solve the defects of the prior art as stated above, one method has been adopted by those skilled in the art whereby an aromatic hydrocarbon solution containing a small quantity of acrylic acid is used and the graft reaction is carried out at relatively low dose rate. However, it is difficult to prepare a homogeneous graft film even when the graft reaction takes place at a low rate of polymerization. And therefore, for preparation of a homogeneous graft film by this method, irradiation with ionizing radiation for a period of from 10 hours up to several days is required, which causes a lowering of efficiency.
As stated above, the prior art methods suffer from the defects that the monomer is not used efficiently, a homogeneous graft film can not be prepared and reproducibility of the graft reaction is invariably poor.
We have carried out a variety of experiments for resolving the defects of prior art as stated above. As a result, we have succeeded in the preparation of a homogeneous graft polymer of low electrical resistivity.